Restore auto-processing of plugins

pkg/analyzer/lib/src/generated/engine.dart

Index: pkg/analyzer/lib/src/generated/engine.dart
diff --git a/pkg/analyzer/lib/src/generated/engine.dart b/pkg/analyzer/lib/src/generated/engine.dart
index aa395b0b13b504ad4af793de68c9c5c74cf4f2ad..1861ffede70a5eabc59870910fc7cc2047832daa 100644
--- a/pkg/analyzer/lib/src/generated/engine.dart
+++ b/pkg/analyzer/lib/src/generated/engine.dart
@@ -824,8 +824,7 @@ class AnalysisEngine {
   TaskManager get taskManager {
     if (_taskManager == null) {
       if (enginePlugin.taskExtensionPoint == null) {
-        throw new IllegalStateException(
-            'The analysis engine plugin has not been registered');
+        processRequiredPlugins();
       }
       _taskManager = new TaskManager();
       _taskManager.addTaskDescriptors(enginePlugin.taskDescriptors);
@@ -858,7 +857,7 @@ class AnalysisEngine {
    */
   void processRequiredPlugins() {
     ExtensionManager manager = new ExtensionManager();
-    manager.processPlugins(AnalysisEngine.instance.requiredPlugins);
+    manager.processPlugins(requiredPlugins);
   }
 
   /**